Diamond-silicon carbide composite and method

ABSTRACT

Uniformly dense, diamond-silicon carbide composites having high hardness, high fracture toughness, and high thermal stability are prepared by consolidating a powder mixture of diamond and amorphous silicon. A composite made at 5 GPa/1673K had a measured fracture toughness of 12 MPa·m 1/2 . By contrast, liquid infiltration of silicon into diamond powder at 5 GPa/1673K produces a composite with higher hardness but lower fracture toughness.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 11/110,252 filed on Apr. 19, 2005, now U.S. Pat. No. 7,060,641,which is a divisional of U.S. patent application Ser. No. 10/448,672filed on May 30, 2003, now issued as U.S. Pat. No. 6,939,506, whichissued on Sep. 6, 2005, all hereby incorporated by reference.

STATEMENT REGARDING FEDERAL RIGHTS

This invention was made with government support under Contract No.W-7405-ENG-36 awarded by the U.S. Department of Energy. The governmenthas certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates generally to composite materials and moreparticularly to diamond-silicon carbide composites with high fracturetoughness and to a method for preparing such a composite.

BACKGROUND OF THE INVENTION

In view of their exceptionally high hardness, excellent wear resistance,and thermal stability, diamond-silicon carbide (SiC) composites havebeen used in various industrial applications such as machining,grinding, drilling, and mining. Diamond-SiC composites have beenprepared by a variety of methods that include chemical vapor deposition,high-pressure high temperature (HPHT) liquid phase sintering, and lowvacuum liquid phase infiltration.

Most available diamond-SiC composites are composed of microcrystallinediamond held together by microcrystalline SiC. Despite theirextraordinary hardness and wear resistance, these composites haverelatively low fracture toughness (<6 MPa·m^(1/2)), which limits theirpotential applications.

Fracture toughness of diamond-SiC composites has been improved byincorporating nanocrystalline diamond into the composites. It isbelieved that the nanocrystalline diamond and SiC hinder dislocationgrowth and microcrack propagation in the composite better thanmicrocrystalline diamond and SiC do, which improves fracture toughness.Such a composite has been reported by E. A. Ekimov, A. G. Gavrilliuk, B.Palosz, S. Gierlotka, P. Dluzewski, E. Tatianin, Yu. Kluev, A. M.Naletov, and A. Presz in “High-Pressure, High-Temperature Synthesis ofSiC-Diamond Nanocrystalline Ceramics,” Applied Physics Letters, vol. 77,no. 7, pp. 954-956). The composite was prepared by the liquid siliconinfiltration of nanocrystalline diamond powder under HPHT conditionsHPHT (7.7 GPa, 1700-2300 K). The composite displayed high fracturetoughness (10 MPa·m^(1/2)) but was not uniformly dense. It was onlypartially densified. The infiltration depth was only 1-2 millimeters(mm) because the pores closed very quickly during infiltration due tothe “self-stop process”; as silicon infiltrates through the pores, itreacts rapidly with diamond to form a silicon carbide phase that sealsthe pores and prevents further infiltration. Alternative methods thatare not limited by the self-stop process may be required to overcomeproblems relating to the self-stop process in order to provide uniformlydense composites with high fracture toughness. Such a method would alsominimize graphitization of nanocrystalline diamond, which has also beena problem in the past.

Uniformly dense, diamond-SiC composites having high fracture toughnessremain desirable.

SUMMARY OF THE INVENTION

In accordance with the objects and purposes of the present invention, asembodied and broadly described herein, the present invention includes aprocess for preparing a uniformly dense diamond-silicon carbidecomposite. The method involves consolidating a powder mixture of diamondand amorphous silicon at a pressure and temperature sufficient toproduce a uniformly dense diamond-silicon carbide composite having aVickers hardness of at least 35 GPa, a Knoop hardness of at least 30GPa, and a fracture toughness of at least 8 MPa·m^(1/2).

The invention also includes a uniformly dense diamond-silicon carbidecomposite. The composite is prepared by a process that includesconsolidating a powder mixture of diamond and amorphous silicon at apressure and temperature sufficient to produce a uniformly densediamond-silicon carbide composite having a Vickers hardness of at least35 GPa, a Knoop hardness of at least 30 GPa, and a fracture toughness ofat least 8 MPa·m^(1/2).

The invention also includes a uniformly dense diamond-silicon carbidecomposite having a Vickers hardness of at least 35 GPa, a Knoop hardnessof at least 30 GPa, and a fracture toughness of at least 8 MPa·m^(1/2).

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthe specification, illustrate the embodiment(s) of the present inventionand, together with the description, serve to explain the principles ofthe invention. In the drawings:

FIG. 1 shows a scanning electron micrograph of ball-milled diamond andsilicon powder;

FIG. 2 shows an x-ray diffraction pattern of the ball-milled powder ofFIG. 1;

FIG. 3 shows a Raman spectrum of the ball-milled powder of FIG. 1;

FIG. 4 a-g shows x-ray diffraction patterns for composites preparedusing the ball-milled powder of FIG. 1;

FIG. 5 a-g shows Raman spectra of the composites having the x-raydiffraction patterns of FIG. 4 a-g, respectively; and

FIG. 6 shows a scanning electron micrograph of the inventiondiamond-silicon composite of EXAMPLE F.

DETAILED DESCRIPTION

The invention includes diamond-SiC composites having high hardness, highfracture toughness, and high thermal stability. The invention alsoincludes a method for preparing the composites. According to theinvention, a homogeneous powder mixture of diamond and amorphous siliconare subjected to a pressure in a pressure range and a temperature in atemperature range sufficient to consolidate the powder into a uniformlydense diamond-silicon carbide composite.

The diamond portion of the powder mixture may be microcrystallinediamond, or a combination of microcrystalline diamond andnanocrystalline diamond. It is believed that in order to produce adiamond-silicon carbide composite with high hardness, a high percentageof microcrystalline diamond is preferable. In order to impart highfracture toughness to the composite, nanocrystalline diamond may also beincluded in the powder mixture. The amount of microcrystalline diamondand nanocrystalline diamond in the powder mixture may be adjusted inorder to provide a composite with high hardness and high fracturetoughness. For producing an embodiment composite, the amount ofmicrocrystalline diamond is from about 60 percent to 100 percent, andthe amount of nanocrystalline diamond is from about zero percent toabout 40 percent.

The powder mixture of diamond and amorphous silicon may be preparedusing a variety of techniques. In each of these techniques, theamorphous silicon is homogeneously distributed around the diamondparticles.

In an embodiment, diamond powder and crystalline silicon powder are ballmilled until the silicon becomes amorphous. The ball-milled powder isthen sintered to form a composite. The ball milling may be performed bya high-energy ball milling technique, or by using a cryogenic millingtechnique at low temperatures of from about 77 degrees Kelvin andhigher.

In another embodiment, diamond powder and silicon powder are jet-milleduntil the silicon becomes amorphous.

In yet another embodiment, the powder mixture of diamond and amorphoussilicon is produced by a thermal technique such as, thermal spray, hightemperature plasma synthesis, and the like.

The amorphous silicon component of the pre-consolidated powder mixturecan be produced from crystalline silicon as the milling technique orspraying technique is applied. Alternatively, amorphous silicon is acommercially available material, and can be purchased and then combinedwith diamond powder to form a mixture. It should be understood thatamorphous silicon is critical for the preparation of composites of thisinvention because amorphous silicon reacts with the surface carbon fromthe diamond powder to form nanostructured silicon carbide, which isbelieved to bond the diamond grains together. It is believed that thisbonding between the grains enhances the fracture toughness greatly. Thereaction between amorphous silicon and the surface carbon of the diamondalso consumes surface defects of the diamond. This also is believed tocontribute to the high hardness of the product diamond-silicon carbidecomposite. Diamond powder generally has defects, and the defects aregenerally concentrated on the surface of diamond grains, so the reactionbetween the amorphous silicon and the diamond removes these surfacedefects as the composite is being formed.

The powder mixture of diamond and amorphous silicon is then consolidatedto form a uniformly dense diamond-silicon composite. A variety oftechniques may be used to consolidate the powder.

In an embodiment, the diamond and amorphous silicon powder mixture isconsolidated by a reactive sintering technique known in the art as theHPHT (high pressure high temperature) technique to form thediamond-silicon carbide composite. A pressure range useful with thistechnique for preparing composites of the invention is a range fromabout 1 GPa to about 10 GPa, preferably from about 3 GPa to about 8 GPa,and more preferably from about 5 GPa to about 7 GPa.

In the HPHT technique, a press is used to consolidate the powder.Presses useful for the HPHT technique include, but are not limited to,multi-anvil presses, toroidal anvil presses, piston cylinder presses,and the like. It should be understood that any press capable ofconsolidating the powder could be used.

In the HPHT technique for powder consolidation, a temperature in atemperature range useful for sintering is from about 800 Kelvin to about2600 Kelvin, preferably from about 1200 Kelvin to about 2400 Kelvin,more preferably from about 1400 Kelvin to about 2200 Kelvin, and evenmore preferably from about 1600 Kelvin to about 2000 Kelvin.

In another embodiment, the powder mixture is consolidated by a techniqueknown in the art as the HIP (hot isostatic pressing) technique. The HIPtechnique involves exposing the powder mixture to a vacuum in a confinedvolume, subjecting the powder to an elevated pressure and an elevatedtemperature.

In another embodiment, the powder mixture of diamond and amorphoussilicon is consolidated by a spraying technique known in the art as“plasma spraying” or “thermal spraying” or “spray coating”. In thistechnique, the powder is applied to a substrate by a high velocity, hightemperature spraying application that results in a uniformly densecoating on the substrate.

A variety of techniques were used to analyze pre-consolidated powder andcomposites prepared from the powder. The morphology and microstructureof the powder was examined using a field emission SEM LEO 1530 apparatus(LEO ELECTRON SPECTROSCOPY, LTD., Cambridge, UK), which has the abilityto minimize the charging effect and take high-resolution pictures underlow working voltage.

The phase composition of the powder and composites was analyzed bypowder x-ray diffraction and Raman scattering spectroscopy. X-raydiffractograms were obtained using a SCINTAG XDS 2000 with a CuK_(α)source. The step was 0.02° and the expose time was 1 second/step duringthe measurement. Raman spectra were obtained using the incidentwavelength of 785 nm. The scattered light passed through a KAISERoptical spectrometer (Ann Arbor, Mich.) equipped with a notch filter,holographic gratings, and a charged-coupled device (CCD) detector. Thespectral resolution was 4 cm⁻¹.

The bulk and shear modulus of the composites were measured on anultrasonic interferometer (AUSTRIALIAN SCIENTIFIC INSTRUMENTS). Toobtain the elastic modulus of each composite, the velocities of thelongitudinal and shear waves were measured using the ultrasonicinterferometer. At least five successive sample echoes were clearlyvisible. Overlap and interference of these sample echoes with those fromthe WC buffer rod enable the determination of the travel time at highfrequencies (20-60 MHz) with a precision of 2×10⁻⁶ seconds (seereference 20).

The densities of composites were measured on AT261 DELTA RANGE (METTLERTOLEDO, Columbus, Ohio) by using the Archimedes method.

The Vickers microindentation hardness was measured on a MICRO4 micronhardness tester (BUEHLER LTD). The load applied to the indenter was 9.8Newtons and holding time was 15 seconds. Twelve indentations were madeto obtain the average hardness value for each composite. The fracturetoughness was measured with a larger applied load (490 Newtons) usingMACROVICKERS 5114 (BUEHLER LTD).

Pre-consolidated powder of the invention was sintered using a tungstencarbide, toroidal high-pressure apparatus (SUPERHARD MATERIALSINSTITUTE, Kiev, Ukraine, references 18 and 19) equipped with ahydraulic press (ROCKLAND RESEARCH, NY). The pressure was calibrated bydetecting phase transitions in bismuth (Bi) and lead-tellurium (PbTe)alloy. The temperature of the cell was calibrated using aW_(3%)Re/W_(25%)Re thermocouple by measuring the temperature near thecenter of the cell as a function of dissipated electric power. Thecalibration curves, pressure versus load, and temperature versus powerwere reproducible, and the estimated maximum deviation of temperaturewas ±100 K, and pressure about ±0.5 GPa. The calibration curves wereused in sintering experiments for temperature and pressure evaluation.

The following EXAMPLES provide details for preparing composites of theinvention (EXAMPLES E, F, and G) and several comparison composites(EXAMPLES B, C, D, H). EXAMPLES A-G employed ball-milled powder. EXAMPLEH describes the preparation of a composite by liquid infiltration,similar to that described by Ekimov (reference 12). The compact ofEXAMPLE A was compressed but not sintered. The parameters of pressure,temperature, sintering time, hardness, and fracture toughness of theproducts of EXAMPLES A-H are summarized in Table 1.

Example A

About 5 grams of synthetic diamond powder having a grain size of about5-10 micrometers (μm) (General Electric Co., Worthington, Ohio) andabout 1.25 grams of silicon powder having a grain size of about 1-20 μm(Alfa Aesar, Ward Hill, Mass.) were mixed together and placed into atungsten carbide vial with tungsten carbide balls. The vial was placedin a CERTIPREP SPEX 8000-D mixer mill (Metuchen, N.J.) inside a sealedglove box that was purged with argon. The powder mixture was ball milledfor about 11 hours. The ball-milled powder was removed and thediamond-to-silicon weight ratio of the ball-milled powder was determinedto be 84:16 by energy disperse x-ray analysis. The powder was furthercharacterized by scanning electron microscopy, x-ray diffraction andRaman spectroscopy.

FIG. 1 shows a scanning electron micrograph (SEM) of the ball-milledpowder. FIG. 1 shows that the ball-milled powder particles have roundshapes and are stuck together.

FIG. 2 shows an x-ray diffraction pattern for the ball-milled powder. AsFIG. 2 shows, the diamond Bragg peaks are sharp and intense. No peakbroadening from strain and size effect was observed. While crystallinesilicon was used to prepare this powder, no diffraction peaks forcrystalline silicon appear in the pattern, which indicates that the ballmilling has transformed silicon from the crystalline to the amorphousstate. Two tiny humps, one at 2θ=28° and the other at 2θ=52°, are barelydistinguishable and indicate that silicon is almost completelyamorphous. No peaks due to tungsten carbide (WC) peaks are apparent inFIG. 2.

FIG. 3 shows a Raman spectrum of the ball-milled powder. The power waskept low, below 10 mW, and the powder was dispersed in water todissipate heat because diamond powder and silicon powder are sensitiveto laser-induced degradation and local heating, especially for grainsizes on the nanometer scale. The spectrum includes Raman peaks fordiamond and amorphous silicon. The diamond F_(2g) peak is located at1332 cm⁻¹. The broad peak at about 500 cm⁻¹ can be explained as asignature of amorphous silicon and can be approximated by a sum of fourGaussian functions that are located at 169, 296, 390, and 474 cm⁻¹ withfull widths at half maximum (FWHM) of 42, 66, 103, and 68 cm⁻¹,respectively. The agreement between the theoretical prediction and theexperimental contour within the frequency region between 200-600 cm⁻¹was excellent.

Together, the SEM, x-ray diffraction pattern, and Raman spectrum ofFIGS. 1, 2, and 3 respectively, provide strong evidence that theball-milled powder is a mixture of crystalline diamond and amorphoussilicon.

The ball-milled powder was placed into the sintering apparatus andcompressed at 5 GPa (±0.5 GPa) at room temperature for about 300seconds. The product was a compact in the form of a tablet with adiameter of 6 millimeters and a height of about 2 millimeters. It had aVickers hardness of 6 GPa and a Knoop hardness of 4 GPa. The fracturetoughness was not measured. The surfaces were ground using a diamondmetal bonded wheel and then polished using 0.5 micron diamond spray and0.1 micron diamond paper. The compact was characterized by x-raydiffraction analysis and Raman spectroscopy. The x-ray diffractionpattern and Raman spectrum of the compact, shown in FIG. 4 a and FIG. 5a respectively, appeared to be almost identical to those for theball-milled powder shown in FIG. 2 and FIG. 3.

Example B

Ball-milled powder, prepared according to the procedure of EXAMPLE A,was placed into the sintering apparatus. The powder was first subjectedto a pressure of 5 GPa (±0.5 GPa) at room temperature within 10 minutes,after which the temperature was increased to 873K within a 3 minuteperiod. The temperature was maintained at 873K (±50K) for 30 seconds,after which the temperature was lowered to room temperature within a 5minute period. The resulting composite was in the form of a tablet witha diameter of 6 millimeters and a height of about 2 millimeters, and hada Vickers hardness of 12 GPa, a Knoop hardness of 10 GPa, a Young'smodulus of 153 GPa, and a fracture toughness of 3 MPa·m^(1/2). Thesurfaces were ground using a diamond metal bonded wheel and thenpolished using 0.5 micron diamond spray and 0.1 micron diamond paper.The x-ray diffraction pattern and Raman spectrum of the composite areshown in FIG. 4 b and FIG. 5 b, respectively.

Example C

Ball-milled powder, prepared according to the procedure of EXAMPLE A,was placed into the sintering apparatus. The powder was first subjectedto a pressure of 5 GPa (±0.5 GPa) at room temperature within 10 minutes,after which the temperature was increased to 1273K within a 3 minuteperiod. The temperature was maintained at 1273K (±50K) for 30 seconds,after which the temperature was lowered to room temperature within a 5minute period. The resulting composite was in the form of a tablet witha diameter of 6 millimeters and a height of about 2 millimeters, and hada Vickers hardness of 14 GPa, a Knoop hardness of 12 GPa, a Young'smodulus of 167 GPa, and a fracture toughness of 3 MPa·m^(1/2). Thesurfaces were ground using a diamond metal bonded wheel and thenpolished using 0.5 micron diamond spray and 0.1 micron diamond paper.The x-ray diffraction pattern and Raman spectrum of the composite areshown in FIG. 4 c and FIG. 5 c, respectively.

Example D

Ball-milled powder, prepared according to the procedure of EXAMPLE A,was placed into the sintering apparatus. The powder was first subjectedto a pressure of 5 GPa (±0.5 GPa) at room temperature within 10 minutes,after which the temperature was increased to 1473K within a 3 minuteperiod. The temperature was maintained at 1473K (±50K) for 5 seconds,after which the temperature was lowered to room temperature within a 5minute period. The resulting composite was in the form of a tablet witha diameter of 6 millimeters and a height of about 2 millimeters, and hada Vickers hardness of 17 GPa, a Knoop hardness of 17 GPa, a Young'smodulus of 410 GPa, and a fracture toughness of 7 MPa·m^(1/2). The x-raydiffraction pattern and Raman spectrum of the composite are shown inFIG. 4 d and FIG. 5 d, respectively.

Example E

The ball-milled powder prepared according to EXAMPLE A was firstsubjected to a pressure of 5 GPa (±0.5 GPa) at room temperature within10 minutes, after which the temperature was increased to 1473K within a3 minute period. The temperature was maintained at 1473K (±50K) for 30seconds, after which the temperature was lowered to room temperaturewithin a 5 minute period. The product was a dense diamond-SiC composite(density 3.3 gram/cm³, porosity 3.5%) in the form of a tablet with adiameter of 6 millimeters and a height of about 2 millimeters. Thesurfaces were ground using a diamond metal bonded wheel and thenpolished using 0.5 micron diamond spray and 0.1 micron diamond paper.The x-ray diffraction pattern and Raman spectrum of the composite areshown in FIG. 4 e and FIG. 5 e, respectively. The composite had aVickers hardness of 36 GPa, a Knoop Hardness of 28 GPa, a Young'smodulus of 507 GPa, and a fracture toughness of 10 MPa·m^(1/2).

Example F

Ball-milled powder prepared according to EXAMPLE A was first subjectedto a pressure of 5 GPa (±0.5 GPa) at room temperature within 10 minutes,after which the temperature was increased to 1673K within a 3 minuteperiod. The temperature was maintained at 1673K (±50K) for 30 seconds,after which the temperature was lowered to room temperature within a 5minutes period. The product was a uniformly dense diamond-SiC composite(density 3.37 gram/cm³, porosity 1.7%) in the form of a tablet with adiameter of 6 millimeters and a height of about 2 millimeters. Thesurfaces were ground using a diamond metal bonded wheel and thenpolished using 0.5 micron diamond spray and 0.1 micron diamond paper.FIG. 6 shows a scanning electron micrograph (SEM) of the polishedcomposite. The x-ray diffraction pattern and Raman spectrum of thecomposite are shown in FIG. 4 f and FIG. 5 f, respectively. Thecomposite had a Vickers hardness of 46 GPa, a Knoop hardness of 40 GPa,a Young's modulus of 564 GPa, and a fracture toughness of 12MPa·m^(1/2).

Example G

Ball-milled powder prepared according to EXAMPLE A was first subjectedto a pressure of 8 GP (±0.5 GPa) at room temperature within 20 minutes,after which the temperature was increased to 2273K within a 3 minuteperiod. The temperature was maintained at 2273K (±100K) for 20 seconds,after which the temperature was lowered to room temperature within a 5minutes period. The product was a uniformly dense diamond-SiC composite(density 3.38 gram/cm³, porosity 0.9%) in the form of a tablet with adiameter of 6 millimeters and a height of about 2 millimeters. Thesurfaces were ground using a diamond metal bonded wheel and thenpolished using 0.5 micron diamond spray and 0.1 micron diamond paper.Compact was then characterized by and x-ray diffraction (FIG. 4 g) andRaman spectroscopy (FIG. 5 g). The composite had a Vickers hardness of45 GPa, a Knoop hardness of 42 GPa, a Young's modulus of 560 GPa, and afracture toughness of 12 MPa·m^(1/2).

Example H

About 100 milligrams of synthetic diamond powder having a grain size ofabout 5-10 micrometers (μm) (General Electric Co., Worthington, Ohio)and about 20 milligrams of silicon powder having a grain size of about1-20 μm (Alfa Aesar, Ward Hill, Mass.) were placed as top and bottomlayers, respectively, in the graphite furnace of the sinteringapparatus. The powder was first subjected to a pressure of 5 GPa (±0.5GPa) at room temperature within 10 minutes, after which the temperaturewas increased to 1673K within a 3 minute period. The temperature wasmaintained at 1673K (±50K) for 30 seconds, after which the temperaturewas lowered to room temperature within a 5 minute period. The productwas a uniformly dense diamond-SiC composite (density 3.4 gram/cm³,porosity 0.6%) in the form of a tablet with a diameter of 6 millimetersand a height of about 2 millimeters. The surfaces were ground using adiamond metal bonded wheel and then polished using 0.5 micron diamondspray and 0.1 micron diamond paper. The composite had a Vickers hardnessof 54 GPa, a Knoop hardness of 52 GPa, a Young's modulus of 575 GPa, anda fracture toughness of 8 MPa·m^(1/2).

TABLE 1 EXAMPLE A B C D E F G H Pressure (GPa) 5 5 5 5 5 5 8 5Temperature (K) room 873 1273 1473 1473 1673 2273 1673 temperatureSintering time (sec) at 300 30 30 5 30 30 20 30 temperature Vickershardness (GPa) 6 12 14 17 36 46 45 54 Knoop Hardness (GPa) 4 10 12 17 2840 42 52 Young's modulus (GPa) — 153 167 410 507 564 560 575 Fracturetoughness — 3 3 7 10 12 12 8 (MPa · m^(1/2))

EXAMPLE A demonstrates the effect of compression alone (5 GPa for 300seconds) on the ball-milled powder. The x-ray diffraction pattern of thecompact, which is shown in FIG. 4 a, is substantially the same as thatfor uncompressed, ball-milled powder shown in FIG. 2. Sharp and intensepeaks from diamond indicate that the microcrystalline state has not beenmodified by compression. Two broad humps at 2θ=28° and 52° indicate thatcompression to 5 GPa has a negligible effect on amorphous silicon (a-Si)crystallization.

The composites of EXAMPLES B, C, and D are non-invention compositesprepared by sintering at 5 GPa/873 k/30 sec, 5 GPa/1273K/30 sec, 5GPa/1473K/5 sec, respectively. The composites of examples B and C areformed at too low a sintering temperature to form a composite with highfracture toughness. The composite of EXAMPLE D, on the other hand, wassintered at a high enough temperature but for too short a time (seeEXAMPLE E). The x-ray diffraction patterns, which are shown in FIGS. 4b, 4 c, and 4 d respectively, show small peaks due to crystallinesilicon that appear in the composite of EXAMPLE B and increase inintensity as the sintering temperature increases from 873K (EXAMPLE B)to 1273K (EXAMPLE C) to 1473K (EXAMPLE D). The formation of crystallinesilicon is also evident from changes in the Raman spectra. The Ramandiamond peak is consistently located at 1332 cm⁻¹, but other spectralfeatures provide evidence of transformation of amorphous silicon intocrystalline silicon, and then into silicon carbide. FIG. 5 a shows theRaman spectrum for the non-sintered composite of EXAMPLE A. As FIG. 5 ashows, the low frequency region of the Raman spectrum is dominated bythe signal for a-Si. As the processing temperature increases from roomtemperature to 873 and then to 1273 and then to 1473, the intensity ofthe a-Si peaks decrease while the intensity of the crystalline siliconpeak at 521 cm⁻¹ increases. It should also be noted that full width athalf maximum (FWHM) of the 521 cm⁻¹ peak decreases from 27 cm⁻¹ to 22cm⁻¹ and finally to 17 cm⁻¹. These changes are believed to indicate anincrease in long-range order of silicon in the corresponding composite.

The invention composite of EXAMPLE E was prepared at the same sinteringtemperature and pressure as that for EXAMPLE D, but the sintering timewas lengthened from 5 seconds to 30 seconds. The Raman spectrum for thecomposite of EXAMPLE E includes a peak at about 800 cm⁻¹ and another atabout 950 cm⁻¹. These peaks are related to nanocrystalline SiC. The peakat about 521 cm⁻¹ is even sharper for the composite of EXAMPLE E thanthat for EXAMPLE D, approaching 10 cm⁻¹ (reference 23). Thesedifferences are consistent with the formation of nanocrystalline SiC,and are supported by the x-ray diffraction pattern of FIG. 4 e, whichindicates the growth of silicon grains.

The invention composites of EXAMPLES F and G were prepared at 1673K and2273K, respectively (higher temperature than for EXAMPLE E). Theresulting composites have what is believed to be the highest fracturetoughness (12 MPa·m^(1/2)) reported for a diamond-SiC composite.

FIG. 6 provides a scanning electron micrograph for the composite ofEXAMPLE F. According to the micrograph, the composite includes 1-10micron sized diamond grains that are surrounded by nanocrystallinegrains of SiC. The Raman spectrum of the composite is shown in FIG. 5 f,and does not appear to include peaks due to silicon, which indicatesthat silicon has reacted completely, or nearly completely, with diamondto form SiC.

The observed x-ray patterns and Raman spectra for the products formedaccording to the EXAMPLES can be explained as follows. Inventioncomposites are not formed at temperatures under 1273K. At 1473K, aninvention composite is not formed if the sintering time is too short (5seconds). According to the invention, a sintering time of about 30seconds at 1473K is long enough for carbon atoms to diffuse from thediamond phase into liquid silicon and react to form SiC. Highertemperatures increase the carbon diffusion rate, leading to consumptionof the silicon.

The composite formed by infiltration (EXAMPLE H) is harder, but has alower fracture toughness than the invention composite of EXAMPLE F.These differences may be explained by the differences in size of the SiCgrains. The infiltration composite includes microcrystalline SiC grainswhereas the invention composite includes nanocrystalline SiC grains. Itis the combination of microcrystalline diamond and nanocrystalline SiCgrains that characterizes the composites of the invention and thiscombination provides composites of the invention with enhanced fracturetoughness. Moreover, the conversion of silicon to the amorphous phaseduring milling appears to be critical to the invention. Amorphoussilicon reacts with diamond to form nanocrystalline silicon carbide,which appears to be inhibited from any further grain growth whensubjected to the sintering conditions described herein.

The foregoing description of the invention has been presented forpurposes of illustration and description and is not intended to beexhaustive or to limit the invention to the precise form disclosed, andobviously many modifications and variations are possible in light of theabove teaching.

The embodiments were chosen and described in order to best explain theprinciples of the invention and its practical application to therebyenable others skilled in the art to best utilize the invention invarious embodiments and with various modifications as are suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto.

1. A method for preparing an uniformly dense diamond-silicon carbidecomposite, comprising consolidating a powder mixture of diamond andamorphous silicon at a pressure and temperature sufficient to produce auniformly dense diamond-silicon carbide composite having a Vickershardness of at least 36 GPa, a Knoop hardness of at least 28 GPa, and afracture toughness of at least 8 MPa·m^(1/2).